Carcinogen-induced mutations to cancer-related genes in target tissues such as bronchial epithelium are thought to be a major mechanism of carcinogenesis. Since detection of mutations in most genes in target tissues is not practical,a surrogate gene which could indicate critical somatic mutations would provide insight into cancer-related mutagenesis. The hypoxanthine phosphoribosyl transferase gene is a good surrogate gene for study,since HPRT mutants are a selectable phenotype which represent a marker for generalized somatic mutation.